The problem is to provide a low electrical resistance connection between a metal and a superconducting ceramic in order to convey high currents at the utilization temperature without disturbing the zero resistance state. Thus, such a connection must have a resistivity of less than 10.sup.-10 .OMEGA..multidot.m.sup.2 for passing a current density of about 10.sup.3 A/cm.sup.2, as is applicable when using a current lead.
At present, good electrical contacts having resistivity of about 10.sup.-10 .OMEGA..multidot.m.sup.2 and used with a YBa.sub.2 Cu.sub.3 O.sub.7-.delta. type ceramic are made by depositing a layer of silver on respective small areas by cathode sputtering. However such a deposit of silver does not adhere well enough to provide a robust and effective connection between the ceramic and a normal metal.
The article entitled "Low resistivity contacts to bulk high Tc superconductors" published on page 2605 of Appl. Phys. Lett. 54 (25), Jun. 19, 1989 describes a method consisting in placing ceramic powder in a mold, in inserting 0.125 mm diameter silver wires orthogonally to the layer of powder, in covering the ceramic powder while allowing the wires to protrude, in compressing the powder, and in baking it. It is observed that such a method is difficult to implement since the silver wires must not be damaged when compression is taking place. In addition, the wires are capable of passing low currents only of about 5 amps.
The above-mentioned article also proposes another method consisting in putting a uniform mixture of ceramic powder and silver powder into a mold, in compressing the mixture, and in baking it at a temperature of 1030.degree. C., which temperature is higher than the melting point of silver. Bubbles are then created, particles of silver disperse, and appear on the surface of the part, thereby defining electrical contacts. The method makes it very difficult to control the shape of these contacts.
The object of the present invention is to avoid these drawbacks and to implement an industrial method capable of ensuring a robust connection with a ceramic.